Polysiloxanes,... preparation and properties by Dr. Salma Amir

polysiloxanes, synthesis and properties

1. Lecture No. 04 Course title: Inorganic Polymers T opic: Polysiloxanes Course instructor: Dr. Salma Amir GFCW Peshawar

2. Polysiloxanes (silicones) Polysiloxanes are the most important class of inorganic polymers. Unlike most other polymers, silicones possess a fully inorganic backbone of -(Si-O)- repeat units. The name silicone usually refers to linear polymers with a silicon-oxygen backbone (-Si-O-) and with alkyl side groups. The most important polyalkylsiloxane possesses only methyl side groups, called polydimethyl siloxane and often abbreviated as PDMS . General structure -[Si(R 2 )-O]- where R = -CH 3 is called poly(dimethyl siloxane )

3. Preparation Commercial silicones are often prepared by hydrolysis of chlorosilanes in the presence of  water. This reaction is exothermic and yields in a first step silanols ,  which are condensed to linear and cyclic oligomers by inter- and intramolecular condensation reactions. Preparation of Monomers Hydrolysis Polymerization

4. 1. Preparation of monomers To prepare alkyl or aryl derivatives of silicon tetrachloride: Examples of such derivatives are RSiCl 3 , R 2 SiCI 2 and R 3 SiCl 2 where R is an alkyl (e.g., CH 3 , C 2 H 5 etc.) or aryl (e.g., C 6 H 5 ) group. The elemental silicon on which the entire technology is based is typically obtained by reduction of the mineral silica with carbon at high temperatures SiO 2 + C Si + 2CO The silicon is then converted directly to tetrachlorosilane by the reaction Si + 2Cl 2 SiCl 4 Alkyl chlorosilanes can also be obtained by the action of Grignard reagent on SiCl 4 RMgCl + SiCl 4 → RSiCl 3 + MgCl 2 2RMgCl + SiCl 4 → R 2 SiCl 2 + 2MgCl 2 3RMgCI + SiCl 4 → R 3 SiCI + 3MgCl 2

5. Method 2 “ Direct Process” or “ Rochow Process ,”which starts from elemental silicon. It is illustrated by the reaction Si + 2RCl → R 2 SiCl 2 Methyl chlorosilanes like (CH 3 )SiCl 3 , (CH 3 ) 2 SiCl 2 and (CH 3 ) 3 SiCl are prepared by heating methyl chloride, CH 3 Cl with Si, catalyzed by Cu, at 300°C. This reaction gives a mixture of methyl chlorosilanes . 2CH 3 Cl + Si Cu (CH 3 )SiCl 3 + ( CH 3 ) 2 SiCl 2 + ( CH 3 ) 3 SiCl 300 C The yield of (CH 3 ) 2 SiCl 2 ( b.p . = 69.6°C) is over 50%. Careful fractionation is used to separate (CH 3 ) 2 SiCI 2 , from (CH 3 )SiCl 3 ( b.p . = 66.9°C) and (CH 3 ) 3 SiCl ( b.p . = 87.7°C ). Compounds of formula R 2 SiCl 2 are extremely important, because they provide access to the preparation of a wide variety of substances having both organic and inorganic character. Their hydrolysis gives dihydroxy structures which condense to give the basic [-SiR 2 O-] repeat unit.

6. 2. Hydrolysis To prepare alkyl or aryl hydroxy derivatives of silicon tetrachloride (called silanols or silandiols ): Examples of such silanols are R Si (OH) 3 , R 2 Si(OH) 2 and R 3 Si(OH). These silanols are obtained by the hydrolysis of RSiCl 3 , R 2 SiCl 2 and R 3 SiCl respectively. RSiCl 3 + 3HOH RSi (OH) 3 + 3HCl R 2 SiCl 2 + 2HOH R 2 Si(OH) 2 + 2HCl R 3 SiCl + HOH R 3 Si(OH) + HCl The general equation representing the hydrolysis reaction can be written as: Chlorosilane + Water hydrolysis Silanols + HCl

7. 2. Polymerization (condensation polymerization) To allow the alkyl or aryl hydroxy derivatives to undergo polymerization: Polymerisation process involves removal of some H 2 O molecules and leads to the formation of different types of silicones. The type of silicone obtained depends on the nature of alkyl or aryl hydroxyderivative and the way in which the hydroxy -derivative undergoes polymerisation . For example:

8. (a) When many molecules of alkyl trihydroxy-silane , RSi (OH) 3 undergoes polymerisation , a cross-linked two dimensional silicone is obtained.

9. Since an active OH group is present at each end of the chain, polymerisation continues on both the ends and hence the length of the chain increases. The increase in the length of the chain produces cross-linked silicone as shown below:

10. (b) When many molecules of dialkyl dihydroxy-silane , R 2 Si(OH) 2 undergo polymerisation , a straight chain (linear) or cyclic (ring) silicone is obtained .

11. Since an active OH group is present at each end of the chain, polymerisation continues and hence the length of the chain increases and gives rise to the formation of long chain silicon, as shown below :

12. (c) When two molecules of trialkyl monohydroxy-silane , R 3 Si(OH) undergo polymerisation , a straight chain silicone (dimer) is obtained.

13. Ring opening polymerization The hydrolysis approach to polysiloxane synthesis has now been largely replaced by ring-opening polymerizations of organosilicon cyclic trimers and tetramers, with the use of ionic initiation . These cyclic monomers are produced by the hydrolysis of dimethyldichlorosilane . Under the right conditions, at least 50 wt % of the products are cyclic oligomers. The desired cyclic species are separated from the mixture for use in ring-opening polymerizations such as those described below.

14. Cyclic siloxanes can undergo a ring-opening polymerization that is a chain-growth process. Free radicals are not useful as initiator species, because of the nature of the siloxane bond, but anionic and cationic initiators are very effective. The reaction is illustrated using the most common cyclic oligomers, the trimer ( hexamethylcyclotrisiloxane ) or the tetramer ( octamethylcyclotetrasiloxane ) (SiR 2 O) 3,4 [-SiR 2 O-] x where R can be alkyl or aryl and x is the degree of polymerization Ring-Opening Polymerizations

15. Properties of P olysiloxanes 1 . T ailoring property The methyl groups along the chain can be substituted by many other groups such as ethyl, phenyl, or vinyl, which allows for tailoring the chemical, mechanical and thermophysical properties for a wide variety of applications.  2. Surface tension Polydimethysiloxanes have a low surface tension in the range of 20 to 25 N/m 2 and consequently can wet most surfaces. With the methyl groups located on the outside, silicones produce very hydrophobic films. Unlike most other polymers, silicones possess an inorganic backbone of -(Si-O)- repeat units. The Si-O bonds are strongly polarized and without side groups, should lead to strong intermolecular interactions. However, the nonpolar methyl groups shield the polar backbone. For this reason, silicone polymers have a very low critical surface tension despite a very polar backbone. In fact, PDMS has one of the lowest critical surface tension of all polymers which is comparable to that of Teflon.

16. 3. F lexibility Due to the low rotation barriers, most siloxanes are very flexible. For example, the rotation energy around a CH 2 -CH 2 bond in polyethylene is about 12.1 kJ/ mol but only 3.8 kJ/ mol around a Me 2 Si-O bond, corresponding to a nearly free rotation. This has far-reaching effects on the thermal and mechanical properties of silicones. Furthermore , chain-to-chain interaction is rather week due to the low cohesive energy, and the distance between adjacent chains is noticeably larger in silicones than in alkanes which also contributes to the greater flexibility of PDMS 4. Viscosity Due to great flexibility of the chain backbone, the activation energy of viscous flow is rather low and they do not become too viscous on cooling, and the viscosity is less dependent on temperature compared to hydrocarbon polymers.

17. 5. Stability Due to the strong Si-O and Si-C bonds, silicone polymers have a very high heat and oxidative stability and outstanding chemical resistance . 6. Diffusibility of gases Due to the large free volume, most gases have a high solubility and high diffusion coefficient in silicones. That is, silicones have a high permeability to oxygen, nitrogen and water vapor, even if in this case liquid water is not capable of wetting the silicone surface! As expected, silicone compressibility is also high . 7. Solubility Many of low molecular weight silicones dissolve in solvents like C 6 H 6 , ether and CCI 4 .